The present invention relates to a white light source having a phosphor mixture provided therein and a liquid crystal display device using the same.
The liquid crystal display device is constituted of a back light unit 1 and a liquid crystal display panel 2, as shown in FIG. 2. The back light unit is further composed of a white light source (cold cathode fluorescent lamp 5 in FIG. 2), an inverter 9 for driving the white light source, a reflector 4, a metal case 3, a diffuser plate 6, prism sheets 7 (7A, 7B) and a reflective polarizer 8.
As the white light source used in the liquid crystal display device, a three band-type cold cathode fluorescent lamp (CCFL) 5 is generally used. The structure of the CCFL is as shown in FIG. 3(a), which shows a sectional view taken along a line parallel to the longitudinal direction of the CCFL. As shown in the figure, the CCFL has a glass tube 11 enclosing mercury gas and a rare gas, phosphor 12 applied onto the inner wall of the glass tube, and electrodes 13 at both ends. The phosphor is a mixture of phosphor powders consisting of a blue phosphor emitting blue light (a main peak wavelength of emission light is about 400 to 500 nm), a green phosphor emitting green light (a main peak wavelength of emission light is about 500 to 600 nm) and a red phosphor emitting red light (a main peak wavelength of emission light is about 600 to 650 nm). One type of phosphor is used per color. Generally, use is made of BaMgAl10O17:Eu as a blue color phosphor, LaPO4:Tb as a green color phosphor, and Y2O3:Eu as a red color phosphor. Note that the former part from the symbol “:” represents a composition of a host material. The latter part from the symbol “:” represents a luminescence center which replaces for a part of the atoms of the host materials. To explain more specifically, in the green phosphor, LaPO4 represents a host material and lanthanum (La) is partially replaced with a luminescence center, terbium (Tb). The CCFL is illuminated by applying voltage to the electrodes 13. When the voltage is applied, mercury atoms are excited within the tube, emitting ultraviolet light. The phosphors are then excited by the ultraviolet light, emitting visible rays. The beam of the visible rays thus emitted passes through the diffuser plate provided immediately upon the CCFL, the prism sheets, and the reflective polarizer, and then enters the liquid crystal display panel. Furthermore, the reflector is arranged to guide light emitted from the CCFL toward the liquid crystal display panel as much as possible.
On the other hand, the liquid crystal display panel has a sectional structure shown in FIG. 4. More specifically, a pair of polarizers 17 and substrates 16 (e.g., glass) are arranged, and liquid crystal 21 and a color filter 22 are sandwiched between the substrates. The liquid crystal are uniformly aligned by the presence of an alignment layer 20 and driven by applying voltage to an electrode group consisting of a plurality of electrodes 18 and formed for each pixel. When voltage is applied to the liquid crystal, liquid crystal molecules are rotated; with the result that refractive index of the crystal changes. In this way, the amount of transmitted light from the back light unit can be controlled. Furthermore, white light W from the back light unit is spectrally separated by color filters into color components, blue light B, green light G and red light R for each pixel and each color filter passes one of the light components therethrough. The liquid crystal display device performs color display by controlling the transmission amount of the light from the back light unit (white light source) for each pixel, thereby dividing the light into light components.
Recently, such a liquid crystal display device has been developed as a device displaying not only a non-moving picture but also a moving picture as in a liquid crystal television. In the development, it has been pointed out that the image of a moving picture looks blurred (referred to as “blurred image”) due to quality deterioration of an image. The blurred image is caused by a hold-type display employed by the liquid crystal display device. To improve this problem, a blink back light system has been proposed (for example, in “Image Quality of Moving Picture in Hold-type Display Device”, Journal of the Institute of Electronics, Information and Communication Engineers, EID99-10, p 55).
The blink back light system is one that light-on and light-off states of a light source are repeated in a single frame (60 Hz is employed in a general liquid crystal display device), as shown in FIG. 5. Usually, the light-on and light-off states are controlled by a timing signal Vsig sent from a circuit integrated in an inverter (FIG. 5(a)). In an ideal light source, brightness varies in the same fashion as the timing signal Vsig changes (FIG. 5(b)). However, actually in a CCFL, the brightness does not sufficiently go along with the change of the timing signal Vsig and follows with a time lag (FIG. 5(c)). This system is greatly expected as a technique for improving the performance of a moving picture in future. If the response characteristics of the CCFL presently in use can be further improved, the performance of a moving picture of a liquid crystal display device can be further improved.
Such a response delay in a CCFL is conceivably caused by delay of an inverter circuit supplying power to the CCFL, response of ultraviolet light generated within the CCFL, and response of phosphors. Of them, the largest rate-determining element is brightness response of a green phosphor. Table 1 summarizes response characteristic evaluation results of phosphor materials presently used in a CCFL. In the table, the brightness response time is defined as the time required until each phosphor material attains 90% brightness based on the largest brightness as being 100%. To explain more specifically, the 90% brightness rise time τon is one required for the brightness to rise from 0% to 90%. The 10% brightness fall time τoff is one required for the brightness to fall from 100% to 10%. As is apparent from the results, the response of a green phosphor, LaPO4:Tb, Ce is particularly slow compared to blue and red phosphors. For example, when a CCFL is illuminated by the half time (Duty 50%) of 1 frame (16.7 msec in the case of 60 Hz), the CCFL cannot go along with a timing signal. Before the brightness of the CCFL completely rises (before the brightness reaches 100%), light-out time (Vsig=0) comes, conversely, before the brightness decreases (before the brightness reaches 0%), light-on time comes (Vsig=V). Therefore, to further improve the quality of a moving picture, it is necessary to attain a high-speed response of a CCFL, that is, a high-speed response of a green phosphor. In addition, a high-speed response of a red phosphor is also desired.
TABLE 1Brightness response time of phosphor materialColorPhosphor materialτon[msec]τoff[msec]BlueBaMgAl10O17:Eu<0.1<0.1GreenLaPO4:Tb,Ce4.75.8RedY2O3:Eu1.81.8
Recently, as a green phosphor material capable of responding at a high speed to possibly overcome this problem, SrAl2O4:Eu is drawn attention. For example, use of such a green phosphor material has been proposed in the following patent documents:
(1) JP-A-8-190894
(2) JP-A-10-49073
(3) JP-A-11-109893
(4) JP-A-2002-313282
(5) JP-A-2001-351578
(6) JP-A-2002-105447
In these documents, high-speed response of a CCFL is attained by using SrAl2O4:Eu alone or in combination with a green color phosphor, LaPO4:Tb, Ce presently in use. When the response characteristics of SrAl2O4:Eu is actually evaluated, both τon, and τoff exhibit less than 0.1 msec. Thus, quite quick brightness response characteristics can be obtained.
However, in applying SrAl2O4:Eu to a CCFL, there are the flowing two big problems. As a result of studies conducted by the present inventors, it is difficult to use SrAl2O4:Eu in practice.
A first problem is a reduction of color reproducibility, more specifically, it is primarily due to a reduction of blue chromaticity. A second problem is a decrease of reliability (brightness deterioration).
The first problem of a reduction of color reproducibility will be explained with reference to FIG. 6. FIG. 6(a) shows emission spectrum of LaPO4:Tb, Ce and SrAl2O4:Eu and spectroscopic characteristics (B_CF, G_CF, R_CF) of a color filter. As is apparent form the figure, the emission spectrum of SrAl2O4:Eu exhibits a very broad profile having a half-band width of about 80 nm with a peak in the vicinity of a wavelength of 520 nm. Since the spectroscopic characteristic of a blue color filter is not sharp, the light of main emission wavelength region of SrAl2O4:Eu can pass through a blue pixel, with the result that the colorimetric purity of blue greatly decreases compared to LaPO4:Tb, Ce, presently in use. The color gamuts of liquid crystal display devices actually using LaPO4:Tb, Ce as a green phosphor and SrAl2O4:Eu are shown in FIG. 6(b). It is found that a chromaticity point v′ of blue is increased by use of SrAl2O4:Eu and shifts toward a green side from the blue chromaticity point defined by NTSC. The human vision is said to be sensible enough to recognize a change of Δ(u,v)=0.2 or more, so that such a shift of blue chromaticity is a significant problem. Furthermore, in the case of JP-A-2002-3132828 proposing use of a mixture of LaPO4:Tb, Ce and SrAl2O4: Eu, it is readily conceivable that a calorimetric purity of blue may decrease by increasing the mixing ratio of SrAl2O4:Eu. To overcome such a decrease of calorimetric purity, it is conceivable that the spectroscopic characteristic of the color filter is made sharp. However, since the color filter is formed of a mixture of a dye and a pigment in order to impart both light resistance and light transmissibity, the profile of spectroscopic characteristics inevitably becomes broad. Therefore, to suppress the colorimetric purity from decreasing, control is desirably made by the emission spectrum of a green phosphor.
The second problem is the reliability of SrAl2O4:Eu. When heat of about 100° C. is applied to this material in an ambient atmosphere containing water or a high-humid ambient atmosphere, the crystal structure of a host material thereof changes, with the result that the brightness significantly decreases. In short, the brightness significantly deteriorates by a CCFL manufacturing process compared to a powdery state at the starting time. In addition, this material has a feature in that it easily absorbs mercury. The material absorbs mercury gas enclosed in a CCFL tube and decreases luminescence efficiency of ultraviolet light. A liquid crystal display device formed of such a chemically labile material cannot ensure the reliability.
As a back light technique for a future liquid crystal TV, the blink system is expected. However, a green phosphor material LaPO4:Tb, Ce presently in use has a problem in response characteristics, whereas SrAl2O4:Eu has big problems in color reproducibility and reliability. In short, a liquid crystal display device satisfying all three characteristics: moving picture characteristics (in particular, CCFL response characteristics), color reproducibility and reliability cannot be obtained by use of conventional techniques.
Hence, an object of the present invention is to provide a moving picture displayable liquid crystal display device capable of attaining satisfactory moving picture characteristic, color reproducibility and reliability at the same time.